gnrc: use new netreg helper functions everywhere

examples/default/main.c

examples/gnrc_networking/udp.c

...patches/0001-RIOT-adapter-port-to-new-netreg-struct.patch

sys/include/net/gnrc.h

sys/include/net/gnrc/conn.h

sys/include/net/gnrc/netreg.h

sys/net/gnrc/application_layer/tftp/gnrc_tftp.c

sys/net/gnrc/application_layer/zep/gnrc_zep.c

sys/net/gnrc/nettest/gnrc_nettest.c

sys/net/gnrc/network_layer/ipv6/gnrc_ipv6.c

sys/net/gnrc/network_layer/sixlowpan/gnrc_sixlowpan.c

sys/net/gnrc/transport_layer/udp/gnrc_udp.c

sys/shell/commands/sc_ccnl.c

sys/shell/commands/sc_icmpv6_echo.c

tests/driver_kw2xrf/main.c

tests/driver_nrfmin/main.c

tests/driver_xbee/main.c

tests/gnrc_sixlowpan/main.c

tests/netdev2_test/main.c

tests/slip/main.c